1. Field of the Invention
The present invention relates to a wavelength converting optical device for obtaining a light source for short wavelengths used for optical information processing or optical measurement equipments and, more particularly, to a wavelength converting optical device using a Cerenkov radiation system.
2. Description of the Related Art
Coherent light sources for short wavelengths have recently been developed for the purpose of application to high-density optical disk systems, measurement/ display systems, and the like. In an optical disk system, since the spot size of a light beam focused on a disk surface is proportional to the wavelength of the light source, a short wavelength light source is indispensable to a high-density optical disk system.
A semiconductor laser as a short wavelength light source is compact and lightweight and has low power consumption. Because of these advantages, semiconductor lasers for shorter wavelengths using new materials have been developed. An InGaAlP semiconductor laser having an oscillation wavelength in the 0.6-.mu.m band (red) has already reached the level of practical application. Although semiconductor lasers for oscillating light beams having shorter wavelengths, such as green or blue light beams, have been studied, no laser capable of continuous-wave oscillation at room temperature has been obtained yet, and hence practical application of such a laser is not warranted.
As another means for realizing a short wavelength light source, second harmonic generation (SHG) using a nonlinear optical crystal is available, and various studies on SHG have been made. Especially, in SHG, in order to realize a compact, low-power-consumption light source, an attempt has been made to form a nonlinear optical crystal into a waveguide by using a semiconductor laser as a fundamental wave light source. For example, a blue light beam (.mu..sub.2) of 1 mW as a second harmonic wave was obtained with respect to a semiconductor laser beam with a fundamental wave (.lambda..sub.1) of 80 mW, by using a proton-exchange LiNbO.sub.3 waveguide having a waveguide portion formed on an LiNbO.sub.3 substrate (T. Taniuchi et al., Extended Abstracts (The 48th Autumn Meeting, 1987); The Japan Society of Applied Physics, 19p-ZG-4). In this system, a second harmonic wave is radiated into a waveguide substrate by Cerenkov radiation. This system is advantageous over the conventional SHG system in that phase matching by angle control, temperature control, or the like is not required.
For practical applications of such a wavelength converting optical element as a short wavelength light source, a light output of at least several mW must be obtained. For this purpose, a light output of 100 mw or more is required as a fundamental wave. As the output of a semiconductor laser is increased, the laser tends to be degraded due to the influences of heat, COD (catastrophic optical damage), and the like. Therefore, a long-term reliability is difficult to ensure. This poses a serious limitation in practical use of a short wavelength light source using the SHG system.
In bulk SHG, an external cavity system is considered as a means for increasing the conversion efficiency from a fundamental wave to an optical second harmonic wave. It is reported that an optical second harmonic wave of 29.7 mW is obtained with respect to, e.g., a light output of 52.6 mW as a fundamental wave by using this system (W. J. Kozlovsky et al., IEEE J. Quantum Electron., Vol-24, No. 6, pp. 913 -919 (1988)). In this system, however, a YAG laser excited by a semiconductor laser is used as a fundamental wave, and the excitation semiconductor laser must have a light output of 500 mW. On the other hand, resonator type SHG in an optical waveguide has been reported. In this case, a conversion efficiency of 0.1% to a second harmonic wave is obtained using Ar laser light as a fundamental wave (R. Reginer et al.: ECOC'86 (1986)). However, in order to realize resonator type SHG in an optical waveguide, both resonation conditions and phase matching conditions must be satisfied. Hence, strict temperature control precision is required. This poses a serious limitation to the practical use of this system.
In an arrangement in which the above-described proton-exchange LiNbO.sub.3 waveguide is used, a Cerenkov radiation beam has a complex wave front due to radiation from axially distributed light sources. More specifically, a Cerenkov radiation beam is constituted by divergent light having different beam waist positions depending on output positions and hence does not have axial symmetry for the case of LiNbO.sub.3 waveguide. For this reason, in order to collimate or focus the radiation beam, a special optical system is required. In addition, it is difficult to decrease the spot size of the beam to a diffraction limit.
In contrast to this, as a system using the same Cerenkov radiation system, SHG by a single-crystal fiber made of a nonlinear material is reported (T. Yamada et al., Extended Abstracts (The 47th Autumn Meeting, 1986); The Japan Society of Applied Physics, 29a-X-2). A waveguide for this SHG has a coaxial structure in which a nonlinear crystal core is covered with a glass cladding. An optical second harmonic wave becomes Cerenkov radiation light propagating in the cladding at a predetermined angle .theta..sub.c with respect to the optical axis, and a beam emerging from a fiber end face becomes a ring-like beam diverging at an angle .theta..sub.0 with respect to the optical axis (.theta..sub.0 =sin.sup.-1 (n.sub.2 sin.theta..sub.c), where n.sub.2 is the refractive index of the cladding with respect to the optical second harmonic wave). This output beam has axial symmetry, but cannot be directly collimated or focused.
As described above, in the conventional SHG system using an optical waveguide for a compact system, a satisfactory conversion efficiency has not yet been obtained, and a light output of 100 mW or more is required as a fundamental wave. In addition, in the system using a resonator type optical waveguide for high efficiency, since strict temperature control precision is required for phase matching, its practical use is difficult.
In the SHG system using Cerenkov radiation requiring no phase matching, it is difficult to collimate or focus the output beam. Moreover, in the fiber system capable of obtaining a beam having axial symmetry, the output beam cannot be directly collimated. Even if collimation is performed by using a special optical system, strict process precision of optical elements is required.